<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html lang="en">
<head>
<title>Source code</title>
<link rel="stylesheet" type="text/css" href="../../../../stylesheet.css" title="Style">
</head>
<body>
<div class="sourceContainer">
<pre><span class="sourceLineNo">001</span><a name="line.1"></a>
<span class="sourceLineNo">002</span>/*<a name="line.2"></a>
<span class="sourceLineNo">003</span> * Copyright (C) 2014 Archie L. Cobbs. All rights reserved.<a name="line.3"></a>
<span class="sourceLineNo">004</span> *<a name="line.4"></a>
<span class="sourceLineNo">005</span> * $Id: CloseableTracker.html 878 2015-04-17 01:32:03Z archie.cobbs $<a name="line.5"></a>
<span class="sourceLineNo">006</span> */<a name="line.6"></a>
<span class="sourceLineNo">007</span><a name="line.7"></a>
<span class="sourceLineNo">008</span>package org.jsimpledb.util;<a name="line.8"></a>
<span class="sourceLineNo">009</span><a name="line.9"></a>
<span class="sourceLineNo">010</span>import java.io.Closeable;<a name="line.10"></a>
<span class="sourceLineNo">011</span>import java.lang.ref.ReferenceQueue;<a name="line.11"></a>
<span class="sourceLineNo">012</span>import java.lang.ref.WeakReference;<a name="line.12"></a>
<span class="sourceLineNo">013</span>import java.util.ArrayList;<a name="line.13"></a>
<span class="sourceLineNo">014</span>import java.util.HashSet;<a name="line.14"></a>
<span class="sourceLineNo">015</span><a name="line.15"></a>
<span class="sourceLineNo">016</span>import org.slf4j.Logger;<a name="line.16"></a>
<span class="sourceLineNo">017</span>import org.slf4j.LoggerFactory;<a name="line.17"></a>
<span class="sourceLineNo">018</span><a name="line.18"></a>
<span class="sourceLineNo">019</span>/**<a name="line.19"></a>
<span class="sourceLineNo">020</span> * Keeps track of {@link Closeable} objects (termed &lt;i&gt;items&lt;/i&gt;) that must be reliably {@linkplain Closeable#close closed}<a name="line.20"></a>
<span class="sourceLineNo">021</span> * prior to shutting down some associated context, but that also must not be closed prior to becoming unreachable, when<a name="line.21"></a>
<span class="sourceLineNo">022</span> * the straightforward strategy of just storing them until context shutdown would require too much memory. In a<a name="line.22"></a>
<span class="sourceLineNo">023</span> * typical scenario, these items are returned (indirectly by reference from some associated &lt;i&gt;holder&lt;/i&gt; object) from some<a name="line.23"></a>
<span class="sourceLineNo">024</span> * method to callers who cannot be expected to reliably close them. Following the simple strategy of just storing all unclosed<a name="line.24"></a>
<span class="sourceLineNo">025</span> * items until context shutdown means unbounded memory use can occur; this class solves that problem.<a name="line.25"></a>
<span class="sourceLineNo">026</span> *<a name="line.26"></a>
<span class="sourceLineNo">027</span> * &lt;p&gt;<a name="line.27"></a>
<span class="sourceLineNo">028</span> * For each such context, an instance of this class may be used to register and track the associated items,<a name="line.28"></a>
<span class="sourceLineNo">029</span> * guaranteeing that they will always eventually be closed, but doing so in a way that avoids memory leaks:<a name="line.29"></a>
<span class="sourceLineNo">030</span> * For each item, there must be a corresponding &lt;i&gt;holder&lt;/i&gt; object containing a reference to it. The holder objects are<a name="line.30"></a>
<span class="sourceLineNo">031</span> * then tracked by this class using weak references. When a holder object is no longer strongly referenced, the corresponding<a name="line.31"></a>
<span class="sourceLineNo">032</span> * item is closed. With this scheme, no memory leak occurs due to this tracking, even when arbitrarily many items are created.<a name="line.32"></a>
<span class="sourceLineNo">033</span> * This of course assumes that when a holder object is no longer referenced, the associated item may be safely closed.<a name="line.33"></a>
<span class="sourceLineNo">034</span> * &lt;/p&gt;<a name="line.34"></a>
<span class="sourceLineNo">035</span> *<a name="line.35"></a>
<span class="sourceLineNo">036</span> * &lt;p&gt;<a name="line.36"></a>
<span class="sourceLineNo">037</span> * A registered {@link Closeable} item is closed at the first occurrence of any of the following:<a name="line.37"></a>
<span class="sourceLineNo">038</span> * &lt;ul&gt;<a name="line.38"></a>
<span class="sourceLineNo">039</span> *  &lt;li&gt;<a name="line.39"></a>
<span class="sourceLineNo">040</span> *  &lt;li&gt;The application itself invokes {@link Closeable#close Closeable.close()} on the item&lt;/li&gt;<a name="line.40"></a>
<span class="sourceLineNo">041</span> *  &lt;li&gt;The associated holder object is no longer strongly referenced, and then {@link #poll} is invoked&lt;/li&gt;<a name="line.41"></a>
<span class="sourceLineNo">042</span> *  &lt;li&gt;{@link CloseableTracker#close} is invoked&lt;/li&gt;<a name="line.42"></a>
<span class="sourceLineNo">043</span> *  &lt;li&gt;{@link CloseableTracker#finalize} is invoked (i.e., this instance is garbage collected)&lt;/li&gt;<a name="line.43"></a>
<span class="sourceLineNo">044</span> * &lt;/ul&gt;<a name="line.44"></a>
<span class="sourceLineNo">045</span> *<a name="line.45"></a>
<span class="sourceLineNo">046</span> * &lt;p&gt;<a name="line.46"></a>
<span class="sourceLineNo">047</span> * Note that {@link Closeable#close Closeable.close()} is required to be idempotent, so application usage<a name="line.47"></a>
<span class="sourceLineNo">048</span> * that results in eager closing of items is appropriate and encouraged. Use of this class may occasionally<a name="line.48"></a>
<span class="sourceLineNo">049</span> * result in {@link Closeable#close Closeable.close()} being invoked more than once for registered items.<a name="line.49"></a>
<span class="sourceLineNo">050</span> * &lt;/p&gt;<a name="line.50"></a>
<span class="sourceLineNo">051</span> *<a name="line.51"></a>
<span class="sourceLineNo">052</span> * &lt;p&gt;<a name="line.52"></a>
<span class="sourceLineNo">053</span> * Instances of this class are thread safe.<a name="line.53"></a>
<span class="sourceLineNo">054</span> * &lt;/p&gt;<a name="line.54"></a>
<span class="sourceLineNo">055</span> */<a name="line.55"></a>
<span class="sourceLineNo">056</span>public class CloseableTracker implements Closeable {<a name="line.56"></a>
<span class="sourceLineNo">057</span><a name="line.57"></a>
<span class="sourceLineNo">058</span>    protected final Logger log = LoggerFactory.getLogger(this.getClass());<a name="line.58"></a>
<span class="sourceLineNo">059</span><a name="line.59"></a>
<span class="sourceLineNo">060</span>    private HashSet&lt;HolderRef&gt; unclosedItems;<a name="line.60"></a>
<span class="sourceLineNo">061</span>    private ReferenceQueue&lt;Object&gt; queue;<a name="line.61"></a>
<span class="sourceLineNo">062</span><a name="line.62"></a>
<span class="sourceLineNo">063</span>// Constructors<a name="line.63"></a>
<span class="sourceLineNo">064</span><a name="line.64"></a>
<span class="sourceLineNo">065</span>    /**<a name="line.65"></a>
<span class="sourceLineNo">066</span>     * Constructor.<a name="line.66"></a>
<span class="sourceLineNo">067</span>     */<a name="line.67"></a>
<span class="sourceLineNo">068</span>    public CloseableTracker() {<a name="line.68"></a>
<span class="sourceLineNo">069</span>        this.reset();<a name="line.69"></a>
<span class="sourceLineNo">070</span>    }<a name="line.70"></a>
<span class="sourceLineNo">071</span><a name="line.71"></a>
<span class="sourceLineNo">072</span>// Public API<a name="line.72"></a>
<span class="sourceLineNo">073</span><a name="line.73"></a>
<span class="sourceLineNo">074</span>    /**<a name="line.74"></a>
<span class="sourceLineNo">075</span>     * Add an item to the set of items being tracked by this instance.<a name="line.75"></a>
<span class="sourceLineNo">076</span>     *<a name="line.76"></a>
<span class="sourceLineNo">077</span>     * @param holder item's holder<a name="line.77"></a>
<span class="sourceLineNo">078</span>     * @param item item to track<a name="line.78"></a>
<span class="sourceLineNo">079</span>     * @throws IllegalArgumentException if either parameter is null<a name="line.79"></a>
<span class="sourceLineNo">080</span>     */<a name="line.80"></a>
<span class="sourceLineNo">081</span>    public synchronized void add(Object holder, Closeable item) {<a name="line.81"></a>
<span class="sourceLineNo">082</span>        this.unclosedItems.add(new HolderRef(holder, item, this.queue));<a name="line.82"></a>
<span class="sourceLineNo">083</span>    }<a name="line.83"></a>
<span class="sourceLineNo">084</span><a name="line.84"></a>
<span class="sourceLineNo">085</span>    /**<a name="line.85"></a>
<span class="sourceLineNo">086</span>     * Poll the internal weak reference queue and close any unclosed items whose holders are no longer reachable.<a name="line.86"></a>
<span class="sourceLineNo">087</span>     *<a name="line.87"></a>
<span class="sourceLineNo">088</span>     * &lt;p&gt;<a name="line.88"></a>
<span class="sourceLineNo">089</span>     * Applications should invoke this method periodically to avoid memory leaks.<a name="line.89"></a>
<span class="sourceLineNo">090</span>     * &lt;/p&gt;<a name="line.90"></a>
<span class="sourceLineNo">091</span>     */<a name="line.91"></a>
<span class="sourceLineNo">092</span>    public void poll() {<a name="line.92"></a>
<span class="sourceLineNo">093</span><a name="line.93"></a>
<span class="sourceLineNo">094</span>        // Poll for refs and close the associated items<a name="line.94"></a>
<span class="sourceLineNo">095</span>        ArrayList&lt;HolderRef&gt; closedItems = null;<a name="line.95"></a>
<span class="sourceLineNo">096</span>        for (HolderRef ref; (ref = (HolderRef)this.queue.poll()) != null; ) {<a name="line.96"></a>
<span class="sourceLineNo">097</span>            if (closedItems == null)<a name="line.97"></a>
<span class="sourceLineNo">098</span>                closedItems = new ArrayList&lt;&gt;();<a name="line.98"></a>
<span class="sourceLineNo">099</span>            final Closeable item = ref.getItem();<a name="line.99"></a>
<span class="sourceLineNo">100</span>            try {<a name="line.100"></a>
<span class="sourceLineNo">101</span>                item.close();<a name="line.101"></a>
<span class="sourceLineNo">102</span>            } catch (Throwable e) {<a name="line.102"></a>
<span class="sourceLineNo">103</span>                this.exceptionDuringClose(item, e);<a name="line.103"></a>
<span class="sourceLineNo">104</span>            }<a name="line.104"></a>
<span class="sourceLineNo">105</span>            closedItems.add(ref);<a name="line.105"></a>
<span class="sourceLineNo">106</span>        }<a name="line.106"></a>
<span class="sourceLineNo">107</span><a name="line.107"></a>
<span class="sourceLineNo">108</span>        // Remove closed items from the set of unclosed items<a name="line.108"></a>
<span class="sourceLineNo">109</span>        if (closedItems != null) {<a name="line.109"></a>
<span class="sourceLineNo">110</span>            synchronized (this) {<a name="line.110"></a>
<span class="sourceLineNo">111</span>                this.unclosedItems.removeAll(closedItems);<a name="line.111"></a>
<span class="sourceLineNo">112</span>            }<a name="line.112"></a>
<span class="sourceLineNo">113</span>        }<a name="line.113"></a>
<span class="sourceLineNo">114</span>    }<a name="line.114"></a>
<span class="sourceLineNo">115</span><a name="line.115"></a>
<span class="sourceLineNo">116</span>    /**<a name="line.116"></a>
<span class="sourceLineNo">117</span>     * Close all unclosed items associated with this instance.<a name="line.117"></a>
<span class="sourceLineNo">118</span>     *<a name="line.118"></a>
<span class="sourceLineNo">119</span>     * &lt;p&gt;<a name="line.119"></a>
<span class="sourceLineNo">120</span>     * The implementation in {@link CloseableTracker} just invokes {@link #reset}.<a name="line.120"></a>
<span class="sourceLineNo">121</span>     * &lt;/p&gt;<a name="line.121"></a>
<span class="sourceLineNo">122</span>     */<a name="line.122"></a>
<span class="sourceLineNo">123</span>    @Override<a name="line.123"></a>
<span class="sourceLineNo">124</span>    public void close() {<a name="line.124"></a>
<span class="sourceLineNo">125</span>        this.reset();<a name="line.125"></a>
<span class="sourceLineNo">126</span>    }<a name="line.126"></a>
<span class="sourceLineNo">127</span><a name="line.127"></a>
<span class="sourceLineNo">128</span>    /**<a name="line.128"></a>
<span class="sourceLineNo">129</span>     * Reset this instance.<a name="line.129"></a>
<span class="sourceLineNo">130</span>     *<a name="line.130"></a>
<span class="sourceLineNo">131</span>     * &lt;p&gt;<a name="line.131"></a>
<span class="sourceLineNo">132</span>     * Results in the forced closing of all unclosed items associated with this instance.<a name="line.132"></a>
<span class="sourceLineNo">133</span>     * &lt;/p&gt;<a name="line.133"></a>
<span class="sourceLineNo">134</span>     *<a name="line.134"></a>
<span class="sourceLineNo">135</span>     * &lt;p&gt;<a name="line.135"></a>
<span class="sourceLineNo">136</span>     * This instance may be reused after invoking this method.<a name="line.136"></a>
<span class="sourceLineNo">137</span>     * &lt;/p&gt;<a name="line.137"></a>
<span class="sourceLineNo">138</span>     */<a name="line.138"></a>
<span class="sourceLineNo">139</span>    public void reset() {<a name="line.139"></a>
<span class="sourceLineNo">140</span><a name="line.140"></a>
<span class="sourceLineNo">141</span>        // Snapshot remaining items and reset state<a name="line.141"></a>
<span class="sourceLineNo">142</span>        final HashSet&lt;HolderRef&gt; itemsToClose;<a name="line.142"></a>
<span class="sourceLineNo">143</span>        synchronized (this) {<a name="line.143"></a>
<span class="sourceLineNo">144</span>            itemsToClose = this.unclosedItems;<a name="line.144"></a>
<span class="sourceLineNo">145</span>            this.unclosedItems = new HashSet&lt;&gt;();<a name="line.145"></a>
<span class="sourceLineNo">146</span>            this.queue = new ReferenceQueue&lt;&gt;();            // discard old queue, everything on it is about to be closed<a name="line.146"></a>
<span class="sourceLineNo">147</span>        }<a name="line.147"></a>
<span class="sourceLineNo">148</span><a name="line.148"></a>
<span class="sourceLineNo">149</span>        // This case happens during construction<a name="line.149"></a>
<span class="sourceLineNo">150</span>        if (itemsToClose == null)<a name="line.150"></a>
<span class="sourceLineNo">151</span>            return;<a name="line.151"></a>
<span class="sourceLineNo">152</span><a name="line.152"></a>
<span class="sourceLineNo">153</span>        // Close unclosed items<a name="line.153"></a>
<span class="sourceLineNo">154</span>        for (HolderRef ref : itemsToClose) {<a name="line.154"></a>
<span class="sourceLineNo">155</span>            final Closeable item = ref.getItem();<a name="line.155"></a>
<span class="sourceLineNo">156</span>            try {<a name="line.156"></a>
<span class="sourceLineNo">157</span>                item.close();<a name="line.157"></a>
<span class="sourceLineNo">158</span>            } catch (Throwable e) {<a name="line.158"></a>
<span class="sourceLineNo">159</span>                this.exceptionDuringClose(item, e);<a name="line.159"></a>
<span class="sourceLineNo">160</span>            }<a name="line.160"></a>
<span class="sourceLineNo">161</span>        }<a name="line.161"></a>
<span class="sourceLineNo">162</span>    }<a name="line.162"></a>
<span class="sourceLineNo">163</span><a name="line.163"></a>
<span class="sourceLineNo">164</span>// Subclass methods<a name="line.164"></a>
<span class="sourceLineNo">165</span><a name="line.165"></a>
<span class="sourceLineNo">166</span>    /**<a name="line.166"></a>
<span class="sourceLineNo">167</span>     * Handle an exception thrown while attempting to {@link Closeable#close close()} and item.<a name="line.167"></a>
<span class="sourceLineNo">168</span>     *<a name="line.168"></a>
<span class="sourceLineNo">169</span>     * &lt;p&gt;<a name="line.169"></a>
<span class="sourceLineNo">170</span>     * The implementation in {@link CloseableTracker} just logs the exception at debug level and returns.<a name="line.170"></a>
<span class="sourceLineNo">171</span>     * &lt;/p&gt;<a name="line.171"></a>
<span class="sourceLineNo">172</span>     *<a name="line.172"></a>
<span class="sourceLineNo">173</span>     * @param item item that was closed<a name="line.173"></a>
<span class="sourceLineNo">174</span>     * @param e exeption thrown<a name="line.174"></a>
<span class="sourceLineNo">175</span>     */<a name="line.175"></a>
<span class="sourceLineNo">176</span>    protected void exceptionDuringClose(Closeable item, Throwable e) {<a name="line.176"></a>
<span class="sourceLineNo">177</span>        if (this.log.isDebugEnabled())<a name="line.177"></a>
<span class="sourceLineNo">178</span>            this.log.debug("caught exception attempting to close " + item + " (ignoring)", e);<a name="line.178"></a>
<span class="sourceLineNo">179</span>    }<a name="line.179"></a>
<span class="sourceLineNo">180</span><a name="line.180"></a>
<span class="sourceLineNo">181</span>// Object<a name="line.181"></a>
<span class="sourceLineNo">182</span><a name="line.182"></a>
<span class="sourceLineNo">183</span>    @Override<a name="line.183"></a>
<span class="sourceLineNo">184</span>    protected void finalize() throws Throwable {<a name="line.184"></a>
<span class="sourceLineNo">185</span>        try {<a name="line.185"></a>
<span class="sourceLineNo">186</span>            this.close();<a name="line.186"></a>
<span class="sourceLineNo">187</span>        } finally {<a name="line.187"></a>
<span class="sourceLineNo">188</span>            super.finalize();<a name="line.188"></a>
<span class="sourceLineNo">189</span>        }<a name="line.189"></a>
<span class="sourceLineNo">190</span>    }<a name="line.190"></a>
<span class="sourceLineNo">191</span><a name="line.191"></a>
<span class="sourceLineNo">192</span>// HolderRef<a name="line.192"></a>
<span class="sourceLineNo">193</span><a name="line.193"></a>
<span class="sourceLineNo">194</span>    private static class HolderRef extends WeakReference&lt;Object&gt; {<a name="line.194"></a>
<span class="sourceLineNo">195</span><a name="line.195"></a>
<span class="sourceLineNo">196</span>        private final Closeable item;<a name="line.196"></a>
<span class="sourceLineNo">197</span><a name="line.197"></a>
<span class="sourceLineNo">198</span>        HolderRef(Object holder, Closeable item, ReferenceQueue&lt;Object&gt; queue) {<a name="line.198"></a>
<span class="sourceLineNo">199</span>            super(holder, queue);<a name="line.199"></a>
<span class="sourceLineNo">200</span>            if (holder == null)<a name="line.200"></a>
<span class="sourceLineNo">201</span>                throw new IllegalArgumentException("null holder");<a name="line.201"></a>
<span class="sourceLineNo">202</span>            if (item == null)<a name="line.202"></a>
<span class="sourceLineNo">203</span>                throw new IllegalArgumentException("null item");<a name="line.203"></a>
<span class="sourceLineNo">204</span>            this.item = item;<a name="line.204"></a>
<span class="sourceLineNo">205</span>        }<a name="line.205"></a>
<span class="sourceLineNo">206</span><a name="line.206"></a>
<span class="sourceLineNo">207</span>        public Closeable getItem() {<a name="line.207"></a>
<span class="sourceLineNo">208</span>            return this.item;<a name="line.208"></a>
<span class="sourceLineNo">209</span>        }<a name="line.209"></a>
<span class="sourceLineNo">210</span>    }<a name="line.210"></a>
<span class="sourceLineNo">211</span>}<a name="line.211"></a>
<span class="sourceLineNo">212</span><a name="line.212"></a>




























































</pre>
</div>
</body>
</html>
